How do I see if a customer has viewed an invoice?

Welcome to the Community space, @BRIAN.

Let me guide you on how to know if a customer has viewed an invoice in QuickBooks Online (QBO).

You can see it under the Invoices page/tab.

1. At the left panel click on Sales
2. Go to the Invoices tab.
3. Click on the Invoice you want to view.
4. It should show as Viewed in the STATUS column. 

When a customer views the invoice, the "Activities" section will show it as "Viewed." You will see this activity by opening the invoice, scroll down, and look for the Invoice activity section.

Moreover, you can also use the   to check the activity history of all transactions.

You can check this photo as your reference:

Also, I've attached some articles you can use to automatically send reminders to customers, as well as some additional information about emailing invoices also, the recording of all the changes made on a specific transaction in QBO:

• Automate invoice reminders in QuickBooks Online
• Email or print multiple invoices and sales forms
• How to view transaction changes in the audit history

Fill me in if you need more guidance about managing your invoices. I'm always ready to back you up. Take the best care!